1. Field of the Invention
The invention relates to a friction roller planetary gearing, including a housing, a sun shaft having a first and a second sun disposed thereon, which are each configured as friction rollers and are axially displaceable with respect to one another and of which at least one is connected for conjoint rotation to the sun shaft, a planet carrier, on which a plurality of stepped planets, which are configured as friction rollers and each have a pair of contact surfaces of smaller diameter and a pair of contact surfaces of larger diameter, are rotatably supported, and an annulus, which is connected to the housing in a manner fixed against rotation relative thereto and is configured as a friction ring, wherein the planets each roll on the suns through the use of the one pair of contact surfaces of the planets and roll on the annulus through the use of at least one contact surface of the other pair of contact surfaces of the planets, and wherein the contact pressure between the suns and the associated contact surfaces of the planets can be controlled by a torque-dependent axial displacement device through the use of a torque-dependent axial displacement of at least one of the suns.
The invention furthermore relates to a speed-changing and differential gearing, including an input stage, a load stage and a differential stage, which are each configured as planetary stages and are disposed coaxially with respect to one another, in particular to a speed-changing and differential gearing for an electrically driven motor vehicle having a housing, a gearing input shaft and two gearing output shafts, which are disposed coaxially with the gearing input shaft, wherein the speed-changing and differential gearing includes an input stage, which is configured as a planetary set and the sun shaft of which, which acts as an input stage input shaft, can be connected as the gearing input shaft coaxially with a drive output shaft of an electric machine and the planet carrier shaft of which acts as an input stage output shaft, a load stage, which is configured as a planetary set and the sun shaft of which is connected as a load stage input shaft coaxially with the input stage output shaft, the annulus of which is fixed in relation to the housing and the planet carrier shaft of which acts as a load stage output shaft, and a differential stage, which is configured as a double planetary set and has a differential input shaft and two differential output shafts, which can be connected as the gearing output shafts to driven wheels of the motor vehicle.
2. Prior Art
Generic friction roller gearings are known from U.S. Pat. No. 3,475,993. Generic speed-changing and differential gearings are found in U.S. Pat. No. 6,401,850. Gearing configurations of coaxial construction and, in particular, coaxial motor/gearing units have proven their worth for electrically driven motor vehicles on the basis of their advantages in terms of installation space, power transmission and efficiency.
There is further potential for improvement in the electric drive unit, i.e. the electric machine as such. It would be advantageous here to be able to use the electric machines employed for very efficient drives of high power to weight ratio in the construction of machine tools. Among the distinguishing features of these machines are very high rated rotational speeds, e.g. higher than 30,000 rpm. It is obvious that gearing structures with high transmission ratios, e.g. those with transmission ratios i=25-40, are required when such high- and very high-speed machines are used in electrically driven motor vehicles. The previously known gearing structures of coaxial construction have transmission ratio ranges which are much too narrow for this purpose. Simply adapting the numbers of teeth and/or the numbers of gearing stages is not a suitable solution to this problem. Owing to the very high rotational speeds, especially those which occur in the input stage of such a gearing configuration, the gearwheels involved would have to be manufactured with such precision that the associated costs would make this solution uneconomical. Moreover, the high rotational speeds would lead to considerable noise pollution, which would not meet modern comfort requirements for motor vehicles.
The use of known friction roller gearings instead of toothed gearings would not help at this point. In particular, a friction roller planetary gearing of the type in question for the occurring rotational speeds and the very large differences in torque between the input and output shaft that are caused by the high transmission ratio would not be suitable for the stared use. The known traction gearing (synonymous with friction roller gearing in the context of the present application) is configured as a planetary gearing with a double sun, symmetric stepped planets and a single annulus fixed in relation to the housing. The sun shaft serves as the gearing input shaft; the planet carrier shaft acts as the gearing output shaft. The suns disposed on the sun shaft are configured as frustoconical friction rollers, the smaller ends of which face one another. A first sun is supported on the sun shaft through the use of a toothing, preventing relative rotation but allowing axial movement. The second sun is supported loosely on the sun shaft. Both suns are in contact through the use of the contact surfaces thereof (these being in the form of frustocone circumferential surfaces) with pairs of correspondingly shaped contact surfaces of the planets, which are configured as friction rollers. Each planet has a circumferential groove beveled on both sides, which separates the individual surfaces of the contact surface pair thereof from one another. The side walls of this groove are in contact with correspondingly shaped flanks of the annulus, which is configured as a friction ring and is fixed in relation to the housing. As is known, a torque is transmitted in such gearings from the input to the output shaft by way of the traction between the various friction elements. The geometrical transmission ratios are calculated in a manner similar to that for toothed gearings by way of the diameters of the participating contact surfaces, wherein in each case the perpendicular distance between the axis of rotation and the actual point of contact on the surface of the friction body is to be taken as the diameter. Thus, in the known traction gearing, the diameter of the planet contact surfaces with the suns is greater than the diameter of the planet contact surfaces with the annulus, with the result that the planets act as stepped planets.
The efficiency of torque transmission through the use of traction depends decisively on the contact pressure of the interacting contact surfaces, while the optimum thereof is very much dependent on the contact surface geometry and on the rotational speeds and torques involved.
The known device therefore has a torque adaptation device configured as a ball-ramp axial displacement device. Thus, a supporting body which guides a set of balls in ball ramps is connected rigidly to the sun shaft on the rear side of the free sun, wherein the balls serve as spacers between the supporting body and the free sun. Depending on the torque to be transmitted, there is a relative rotation of the free sun and the supporting body, this being converted through the use of the ball ramp guide into an axial displacement of the free sun. This results in an increase in the contact pressure between the suns and the planets, but this is also associated with a radial displacement of the planets which, for its part, leads to a change in the contact pressure between the planets and the annulus. The radial displacement of the planets is made possible, in particular, by the fact that the bearing journals on which the planets are supported are supported in radial slots in the planet carrier.
This embodiment of a traction stage has various disadvantages. On the one hand, the radially displaceable support of the planet bearing journals is technically complex and therefore expensive. Secondly, as described, a torque-dependent change in the contact pressure between the suns and the planets also has a direct effect on the contact between the planets and the annulus. This is disadvantageous, especially in the case of high transmission ratios, where the rotational speed and torque ratios in the various contact areas differ widely, because an optimum setting in both contact areas is impossible as a result. Finally, it must be regarded as a disadvantage, especially in the case of high-speed applications, that centrifugal forces acting on the planets also act directly on the contact between the planets and the annulus (imposing a load) and between the planets and suns (with a load-relieving effect), owing to the radially displaceable support of the planet bearing journals.